Collimated electric arc-powder deposition process



Jan. 9, 1962 R. M. GAGE ETAL 3,015,447

COLLIMATED ELECTRIC ARC-"POWDER DEPOSITION PROCESS Filed Nov. 2, 1959 40 0 4 i J 54 2 3 4 J15 Z 4 K 3?. l R E 5 m 0 \E W 4 H 01 1 /R% R P i; mm Ma 4 w wu m a 4\ 2 6\ w i K E M I O w msm DAT w Wx.

POWDER 8- GAS INVENTORS ROBERT M. GAGE ONTARIO H- NESTOR DONALD M-YENNIQ WJZQ A T TORNEV United States i is This invention relates to the artof electric arc fusing and welding particles of powder together, andmore particularly to building up a mass on a suitable base or workpieceby spray-casting powdered material thereon by means of a collimatedarc-efiiuent or plasma.

The present application is a continuation-in-part of applications,Serial No. 706,099 and Serial No. 706,135, filed December 30, 1957;which applications are, in turn, continuations-in-part of applicationsSerial No. 631,557 and Serial No. 631,558, filed December 31, 1956,respectively, all now abandoned.

Space missiles equipped with graphite exhaust nozzles, for example, canhave the linings of such nozzles coated with tungsten metal according tothe invention here proposed to provide one of the greatest advances inthe rocket art to date.

Heretofore, many processes have been proposed for producing coatings onsurfaces of workpieces. Some of such processes employed oxy-fuel flamesto fuse a stream of powder or a rod, and the fused particles weredeposited on the surface to be coated. Such processes presented aninherent disadvantage in that the temperatures developed by a flame werenot sufficiently high to fuse the higher melting point coatingmaterials. Another disadvantage was that in such processes the controlof the ambient coating atmosphere was narrowly limited to the oxidationor reduction potential of the flame chemistry. Even then, desiredvariations in oxidation or reduction potential of the coating atmosphererequired complex adjustment of fuel and oxidant feed withthe usualundesirable resulting variation in flame temperature. Neutral or inertcoating atmospheres were essentially impossible to attain when desired.

Coating processes also have been suggested employing electric arcs. Someof these involved the use of a gasshielded, non-consumableelectrode-electric torch and a separate filler rod of the surfacingmaterial. A disadvantage of such processes is that the deposited coatingmaterial alloys to a substantial degree with the base material withoutany ability to control the degree of this alloying. In addition, aconsiderable degree of skill is required by the operator in suchsurfacing processes.

Various other metallizing processes employing conventional electric arcshave been proposed, but the thermal energy concentrations anddistributions, the energy transfer of the arc to the coating material,and the acceleration of the coating particles are usually insufficientto heat and propel the material to the extent required to form sound,dense, adherent coatings on the surface of the workpiece.

It is, therefore, the prime object of the present invention to provide amethod for coating surfaces of a workpiece employing electric arcs whichaccomplishes the transfer of a sufficient amount of energy to thecoating material to insure the formation of sound, dense, adherentcoatings.

Another object is to provide a method for depositing material on aworkpiece in a manner to effect a controlled degree of fusion, forexample to another workpiece, wherein an electric arc is employed whichis capable of transferring a sufficient amount of energy to the materialto be deposited to insure substantial fusion thereof and wherein adeoxidizer may be included, if desired.

The present invention provides a novel process of'depositing powderedcoating material which involves passing such material in the form ofrelatively fine powder into, axially through and with an arc and ionizedinertgas stream of a collimated Gage-type high-pressure are which iswal-stabilized through lateral constriction, and depositing theso-accelerated and heated particles on a suitable base by directing theso-collimated efiiuent containing such material against such base. a

in accordance with the present invention, a method is provided fordepositing material on the surface of a suitable base or workpiececomprising concurrently forming a high-pressure electric are between anon-consumable stick electrode and a second electrode spaced therefrom,passing a stream of gas in contact with the stick electrode to containthe arc, construing the arc-containing gas stream and wall-stabilizingat least a portion of the arc to collimate the energy of the arc andproduce a high thermal content effluent, passing coating material in theform of powder through and with the high thermal content effluent toheat and propel the material, and depositing the resulting hot materialon such surface. When the method of the invention is employed to effectthe coating of a surface of a workpiece, the deposited material forms asound, dense, adherent surface, consisting of irregular shapedmicroscopic leaves interlocked and welded with and to each other andsuch surface.

In one species of the invention the arc is transferred to the base orwork which is in the electrical circuit. This species employs an arc ofthe type described in US. Patent 2,806,124. Another species of theinvention employs a non-transferred arc wherein the base or work is notin the electrical circuit. This species employs an arc of the typedescribed in US. Patent 2,858,411. The non-transferred arc species ispresently preferred for coating operations, while the transferred arcspecies is preferred for fusion bonding and welding operations whereincontrolled degree of fusion is desired between the heated particles andthe base or workpiece.

The process of the present invention is capable of spraycasting such lowmelting point materials as iron, aluminum, nickel, and the like, as wellas relatively higher melting point materials such as tungsten.molvbdenum. tu gsten carbide, hafnium carbide, and the like. Sometimesbinders, such as iron, nickel, and cobalt, in amouts up to approximately30 percent by weight may be added to the material. It is necessary,however, that at least a substantial portion of the coating mixturebecomes plastic during the coating process in order to attain dense,adherent coatings.

It has been found that many gases can be employed in this processdepending on the type of material being coated and on the coatingdesired. If a pure metal coating is desired, an atmosphere inert both tothe coating material and the base or work, such as argon, helium, and insome circumstances nitrogen, hydrogen, or carbon monoxide, should beemployed. Mixtures of gases, such as argon-nitrogen and argon-hydrogen,can also be used when desired to deposit the desired material. The practical advantage in this regard of the present are torch plating processis that the chemistry of the ambient atmosphere can be controlledwithout basically affecting the energy available for heating the coatingmaterial. One item which must be considered in the choice of a givenatmosphere (gas) is that precautions be taken to prevent damage to thetorch.

The coating material in the form of powder may be introduced into thecollimated arc zone directly. It may also be introduced into gas whichacts to carry the material into the arc.

For some materials it may be introduccd into and with the high thermalcontent .efl luent.

It has been found that a direct current source of electric power ateither reverse or straight polarity, or an alternating current powersource, may be employed to energize the are in accordance with theprocess of the invention.

In the drawings:

FIG. 1 is a schematic view, partly in "vertical crosssection, ofapparatus for carrying out the invention with the work-out-of-circuit(non-transferred);

FIG. 2 is a similar view of a modification thereof;

FIG. 3 is a schematic View, partly in vertical crosssection, ofapparatus for carrying out the invention with the work-in-circuit(transferred); and

FIG. 4 is a similar view of a modification thereof.

Referring to FIG. 1, a non-transferred arc torch is provided having aninternal stepped-diameter cylindrical bore 12 terminating at the lowerend in an arc stabilizing orifice 14 and having axially positionedtherewith a non-consumable stick electrode 16, of tungsten or the like.The stick electrode may contain highly emissive material such as thoria.Inlet means 18 is provided for introducing water into an annular space20 to cool the lower end of the torch, and outlet conduit 22 is providedto carry the cooling water from the torch body. Stick electrode 16 iselectrically insulated at 28 from the upper end 30 of torch 10 and isconnected, through line 32, to the negative terminal of a direct currentpower source 34. Line 35 connects the positive terminal of power source34 to the torch body 10.

A powder and gas inlet conduit 38 is provided in torch body 10 andcommunicates with the interior of boring 12 to introduce apowder-carrying gas stream into the annular space formed between thewalls of bore 12 and sleeve 11. Additional torch gas enters throughconduit 46 in the upper end of torch body 10 and passes through annularspace 48 to shield the stick electrode 16. Such gas flows from annularspace 48 around the end portion of the electrode 16 while the gas-bornepowder stream entering through annular space between thesleeve 11 andthe inner cylindrical wall of torch body 10 is thereby shielded fromundesirable contact with the stick electrode.

As the are 40 is established, with the gas stream, the arc effluent orplasma conforms to the shape of the arc stabilizing orifice 14, and acollimated arc is produced and the powder carried by the gas streampasses axially through such high-energy collimated arc, is heated andpropelled by the high thermal content efiiuent (plasma), and isdeposited as a dense, adherent coating 42 on the surface of workpiece44. The gas flow through orifice 14 is preferably in an axial direction.

The apparatus of FIG. 2 is a double-anode modification of that shown inthe embodiment of FIG. 1. In this modification a ballast resistor 50 isprovided between the torch body (secondary electrode) and the positiveterminal of power source 34 to maintain a pilot are between theelectrode 16 and the torch body 10; and a remotely-positioned primaryelectrode 52, having an orifice 54 and cooling fluid inlet 56, coolingfluid passage 58 and cooling fluid outlet 60, is connected directly tothe positive terminal of the power source. Coating material in the formof'powder suspended in a gas stream may preferably be introduced throughthe annular space between bore 12 and sleeve 11.

A Wide variety of workpieces have been satisfactorily coated with a widevariety of powdered materials, both with and without binder components.

In addition to coating operations, suitable deoxidation powders may beintroduced into a treating zone in accordance with the process of theinvention, or fusion of two workpieces such as in a welding opera i n ybe effected in accordance with the process of the invention. In carryingout such processes, the degree of fusion between the welded particlemass and the workpiece is controlled by varying the thermal energysupplied to the workpiece.

The following table sets forth operating conditions obtained inproducing three coatings of tungsten carbide- 8 percent cobalt material(325 x D mesh) for a x x 2-inch plain steel bar traversed under thetorch shown in FIGURE 2.

TABLE I Anode Nozzle 1 Size, inches Coating Thickness Torch to WorkDist., inches Surface Speed, i.p.m.

c.f.h. Argon Powder low, g.p.m.

Amps. Volts xxx ,5 inch diameter, spacing between nozzles,

TABLE II Non-transf rred. arc torch coatings on steel Powder Flow,g.p.m.

Torch- Work Spacing, inches Coating Volts c.t'.h.

Argon Amps Size,

inches A1203 Cr m-F394 Stainless steel 301 stainless steel. Al

TiIIIIIIIIIIIIIIII The high-pressure arc plating process of theinvention is extremely versatile. A wide variety of materials can becoated under diverse conditions by a proper combination of processvariables. The energy available for heating can be controlled by varyingthe arc current, torch gas and gas flow rate and orifice size; coatingparticle velocity can be varied by gas flow rate and powder particlesize; the chemical character of the coating atmosphere and resultingcoating can be controlled by varying the composition of the gasemployed. In order to obtain acceptable coatings, the current densityand gas velocity are maintained at relatively high values. The widevariety of controllable process variables thus serves to impart extremeversatility to the process of the invention.

It has been found that for nozzle orifice should be at least A inchdiameter. 'When orifice sizes smaller than this are used, it becomesdifficult to properly maintain an arc in the orifice. Such small nozzlesare also subject to plugging by the coating material. When tungstenpowder, for example, is used as the coating material, it has beenconvenient to use nozzles as large as inch diameter.

For practical purposes the nozzle orifice should not be larger thanabout /2 inch in diameter, otherwise the extremely large arc currentsthat will be necessary to substantially fill the orifice passage willprovide an undesirably large overall efiiuent heat output which couldharm the workpiece being coated.

The total collimated are energy to which the coating material is exposedin the torch orifice passage is an important variable forsuccessful'coating application. Relatively low melting point materialsor materials of practical purposes the torch V fine particle sizerequire a lower total energy for proper coating conditions as comparedwith higher melting point or larger particle size materials. One methodof varying the are energy is to vary the arc current. However, onlylimited control is thus available since the useful range of arc currentis related to the orifice size being used.

Too low a current value result in a non-collimated arc with itsattendant lower arc intensity, while too high a current will damage thegiven orifice passage. With a /8 inch diameter orifice, the arc currentshould be at least 30 amperes, for example, for a useful coating oftungsten metal using a powder having an average particle size of 4.5microns. The preferred lower limit for are current in a /8 inch diameterorifice passage is about 100 amperes for a tungsten coating. No absoluteupper limit has been found, but currents as high as 400 amperes havebeen tested successfully in orifice passages up to about inch diameter.It has generally been found that as the arc current increases in a givenapparatus, the resulting coating porosity also decreases.

An alternative and preferred method of varying the total are energyavailable to the coating material is to vary the dwell time that a givencoating particle is exposed to the collimated are. One method ofattaining this result is to vary the length of the nozzle orifice.Orifice lengths from about A; inch to about 1 inch could conveniently beused. Desirable coatings of tungsten are obtained, for example, using ainch diameter orifice 4 inch long with a 200 ampere are and 300 c.f.h.argon gas flow through the orifice. Alumina coatings are obtained undersimilar conditions using a A; inch long orifice.

Another method of varying the dwell time of the coating particle in theorifice is to vary the gas velocity and thus vary the particle velocity.The important criterion is that sufiicient gas velocity is attained toproperly accelerate the coating particles so as to form a dense,adherent coating. In addition, when a single nozzle electrode torch, ofthe type shown in FIG. 1 is used, the gas velocity in the chamberbetween the stick electrode and nozzle electrode must be sufficientlyhigh to force the are down into the nozzle orifice. if at any time thearc is permitted to go directly to the side of the nozzle electrodemouth, then plating ceases since the bulk of the gas and coatingmaterial can pass around the arc and out through the orifice withoutbeing heated sufficiently for proper plating.

It has been found that the gas flow should be at least 20 c.f.h. throughnozzles of the above description. No upper limit has been found butflows as high as 380 c.f.h. have been successfully employed. As the gasflow increases, the coating porosity generally decreases. It isdesirable that the powder velocity leaving the orifice passage be atleast 500 fps. and preferably at least 1000 fps.

The rate at which a coating is deposited is dependent upon the coatingmaterial feed rate as well as the general coating conditions. As low as0.6 gram/min. and as high as 85 grams/min. have been used with Ms-V inchdiameter orifices. Such feed rates are not limiting since higher orlower amounts could be used if desired. Deposition rates of the coatingon the workpiece have varied from 0.1 gram/min. to above 20 grams/min.

Another variable which has an effect upon coating quality is thestand-elf distance between the torch orifice outlet and the baseplate.Stand-off distances of about Vs to 4 inches have been used with about 2inches being conveniently used for most coating applications. At lowstand-off distances undesirable baseplate heating results. At higherstand-off values the hot torch effluent gases can cool somewhat toreduce the amount of heat reaching the workpiece without substantiallyreducing the coating material temperature. However, at such high stand-6a values the coating particles have a tendency to be oxidized by aircontamination.

This latter problem is alleviated by using a protective extension in theform of cylindrical barrel 64 below the torch orifice as shown inFIG. 1. Such torch extension should have an internal diameter at leastabout three times the diameter of the nozzle orifice passage in order toprevent coating buildup along the extension wall. Such protectiveextension is necessary only when oxidizable coating materials are used.It is helpful but not necessary for other coating materials.

The coating material must be of such size that it can be uniformlydispensed. Also it should be as fine as possible, compatible withuniform dispensing, so as to permit rapid heating and acceleration inthe arc stream. Powders of 325 mesh and smaller are quite satisfactory.

The present invention has been found quite useful to deposit refractorymetal coatings, for example metallic tungsten. Prior methods have beenincapable of producing such metallic coatings due to the high meltingpoints of the metals. Recently, however, coatings of refractorymaterials were found obtainable by the employment of the method ofUnited States Patent 2,714,563 wherein a detonation phenomenon isemployed to partially melt and impart velocity to powder suspended in adetonated body of gas. When tungsten is applied by such detonationprocess, the metal particles have a tendency to react with the productsof combustion (detonation) and, depending on the oxygen-fuel ratio, willbe oxidized or carburized. The resulting coating, therefore, is not ametal of'the purity required for uses such as electron emittingelements, high temperature filaments, and the like.

Pure metallic coatings can be produced in the practice of the process ofthe present invention and such coatings are dense and adherent to anextent not obtained heretofore.

In one example, a metallic tungsten coating was deposited employingapparatus similar to the type shown in FIG. 1. An are between the stickelectrode and the nozzle electrode was maintained at 250 amperes. Argongas at c.f.h. and tungsten powder (average particle size 4.5 microns) atabout 6 grams per minute passed axially down through the /8 inchdiameter anode throat. The resulting hot gas-coating particle stream wasdirected.

against a steel workpiece to deposit a tungsten coating havingmicroscopic porosity below 1 percent and a VPN hardness of 470(equivalent to swaged tungsten).

Similar equipment was used again at 55 volts (DCSP) and amperes with agas mixture of 38 c.f.h. argon and 65 c.f.h. nitrogen passing through a$1 inch diameter nozzle anode. The tungsten coating had 3 percentporosity and VPN hardness of 320.

In another example an arc of 200 amperes and 65 volts (DCSP) wasmaintained between a A; inch diameter tungsten cathode and awater-cooled copper nozzle having a A; inch diameter nozzle passage. Thetungsten cathode was surrounded along part of its length by a cooledcopper shielding gas sleeve. Argon gas at c.f.h. passed down along thetungsten cathode in the annular space between the shielding gas sleeveand the cathode. A second argon stream of 150 c.f.h. carrying 30grams/min. finely-divided tungsten powder (average particle size of 6microns) was introduced along the annular space surrounding theshielding gas sleeve. The total gas and powder flow passed concurrentlythrough the nozzle passage where they were heated by the collimated arcand then passed through a nozzle extension tube inch ID. and 2 incheslong attached to the nozzle anode. The hot tungsten particles were thenimpinged on a workpiece positioned near the outlet of the nozzleextension tube to form a dense, adherent coating. The as-depositedcoating had a modulus of rupture of 50'65,000 p.s.i.; modulus ofelasticity of 22x10 p.s.i.; density about 90% of theoretical; and ahardness of 400-470 VPN. After heat treatment at U 1600 C., the coatinghad a modulus of rupture of 68,000 psi; modulus of elasticity of 50x10p.s.i.; and a density of about 93% of theoretical.

In another example, a metallic tungsten coating was deposited employingapparatus similar to the type shown in FIG. 2 of the drawing except thatbarrel 64 was omitted. The torch was operated at 150 amperes and about40 volts (DCSPL while argon gas at 120 c.f.h. was passed axially throughthe nozzle of the torch. Tungsten powder particles having a size below11 microns was introduced into the argon stream at 2 grams per minute toproduce a substantially pure tungsten metal coating on a copperworkpiece. 1

In another example, the same apparatus was employed with a barrel 64consisting of a A inch I.D. extension tube 1 inch long' attached to thelower anode as shown in FIG. 2. A steel workpiece positionedapproximately inch from the end of the extension tube was rotated duringthe plating operation. The torch was operated at 47 volts (DCSP) and 125amperes while argon gas at 150 c.f.h. was passed through the torchnozzle. Tungsten powder of 4.5 microns (average particle size) was introduced into the argon stream at a rate of 5 grams per minute. Theresulting tungsten metal coating had an excellent interface bond,porosity was less than 1 percent, and the hardness approached that ofcold drawn tungsten rod (450 Diamond Pyramid hardness).

In still another example apparatus similar to that shown in FIG. 1 wasused. Argon gas at 162 c.f.h. passed down along the tungsten cathode inthe annular space between the shielding gas sleeve and the cathode. Asecond argon stream of 132 c.f.h. carrying 20 grams/min. finely-dividedhafnium carbide powder was introduced along the annular spacesurrounding the shielding gas sleeve. The total gas and power flowpassed concurrently through the nozzle passage where they were heated bya 200 ampere- 59 volt direct current collimated and wall-stabilized areand then passed through a nozzle extension tube about 2 inches longattached to the nozzle anode. An additional nitrogen gas stream of 30c.f.h. was introduced near the outlet of the extension tube to aid inshielding the hot coating particles from atmospheric contamination. Thehot hafnium carbide particles were then impinged on a /2 inch diameterbrass tube workpiece to form a dense adherent coating 0.015 inch thick.

A wide variety of other refractory metals, such as molybdenum, tantalum,columbium, rhenium, and the like, may be applied as coatings inaccordance with this aspect of the invention.

Referring to FIG. 3 of the drawing, a transferred arc torch 10 isprovided having an internal stepped-diameter cylindrical bore 12terminating at the lower end in an arc stabilizing orifice 14, andhaving axially positioned therewith a non-consumable stick electrode 16,of copper or the like. Inlet pipe 18 is provided for introducing waterinto the annular space 20 to cool the lower end of the torch, and outletpipe 22 is provided to carry the cooling water from the torch body.Similarly, inlet conduit 24 is provided for introducing cooling waterinto the body of stick electrode 16, and outlet conduit 26 provided forconducting such water therefrom. Stick electrode 16 is electricallyinsulated at 28 from the upper end of torch 10. Electrode 16 isconnected through conductor 32 to the positive terminal of a directcurrent power source 34. Conductor 36 connects the negative terminal ofpower source 34 and the base or workpiece 44 to be coated.

A ballast resistor 54 is also electrically connected between thenegative terminal of power source 34 and the torch body 10. This servesto continuously maintain a pilot are between the stick electrode 16 andthe torch body 10. In case the main are 40 between the electrode 16 andthe workpiece 44 is interrupted, the pilot arc aids in reestablishingthe main are.

A powder and gas inlet conduit 38 is provided at the upper end of torchbody 10 and communicates with the interior of bore 12 to introduce thepowder-carrying gas stream into the annular space 12a formed between thewalls of bore 12 and the outer surface of stick electrode 16. As themain are 40 is established, with the gas stream, the arc efliuent orplasma conforms to the shape of the arc stabilizing orifice 14, and acollimated arc is produced; the powder carried by the gas stream passesaxially through and along with such high-energy collimated arc, isheated and propelled thereby and is deposited as a dense adherentcoating 42 on the surface of base or workpiece 44. The gas flow throughorifice 14 is preferably in an axial direction.

The embodiment of apparatus shown in FIG. 4 of the drawing differs fromthat of FIG. 3 in that water-cooled copper electrode 16 is provided witha tungsten tip 46, and that the gas introduced through conduit 48 toannular space 50, and around the electrode tip 46, mainly provides thearc with gas that is ionized by enveloping such electrode tip 46, whilethe gas-borne powder stream entering through inlet conduit 42 to annularspace 12a is thereby shielded from undesirable contact with suchelectrode tip 46. A tungsten stick electrode may be substituted for thetipped copper electrode 16, if desired. The tungsten electrode may alsocontain emissive material such as thoria. Another difference between theapparatus of FIG. 3 and that as shown in FIG. 4 is a reversal of thepolarity of the direct current power source employed. A nozzle extension 64 similar to that shown in FIG. 1 can also be used with theapparatus of FIGS. 3 and 4.

A wide variety of workpieces in the are circuit have been satisfactorilycoated according to invention with a wide variety of coating materials,both with and without binder components. The following table sets forthresults for a few of such coating operations employing a direct current,reverse polarity power source in the manner shown in FIG. 3 of thedrawing and an arc torch operated at V3 inch from A1 inch thickworkpieces of the composition set forth. The resulting coatings were alldense and adherent, composed of irregularly shaped microscopic leaveswelded in interlocking relation with one another.

TABLE III Powder Are Power Trav- Feed Argon erse Powder Rate, WorkpieceFlow, Speed, g'n/ Volts Amps c.t.h. i.p.m. mm

WC+8% (30.. 2 Steel 28 40 15 WC+8% O0" 2 Aluninun'L- 28 80 40 15 WC|8%Co 2 Carbon 28 80 40 15 WO+8% C0 2 sttstinlpss 30 80 40 20 eeAluminum..- 1-4 Steel 28 80 4O 15 Iron 1-4 do 28 80 40 15 C Carb0n 35-40250 25 8 1 35-40 240 25 8 Stainless Steel. 3 Steel 26 80 40 13 Do 3Aluminum 26 80 40 13 Enamel frit..- ElteeL 50 HastelloyG. 3 do 24 80 4013 Do 3 Aluminum 24 8O 40 13 Hastelloy 75 K 3 Steel 25 8t) 40 1 3 TheHastelloy powder materials employed in the last three examples arenickel base alloys produced and sold by Haynes Stellite Company,Division of Union Carbide Corporation. 3

In addition to coating operations, suitable deoxidation powders may beintroduced into a treating zone in accordance with the process of theinvention, or fusion of two workpieces, such as in a welding operation,may be effected in accordance with the process of the invention. Thefollowing table sets forth the data for two such deoxidation and onesuch metal fusion applications employing apparatus and torch spacingconditions substantially identical with those employed in the examplesof Table III.

Whereas argon was employed as the carrier and areenveloping gas in suchexamples, a wide variety of other gases such as hydrogen, helium, carbonmonoxide, carbon electrical circuit. The employment of relatively lowcurrents, such as about 80 amperes and gas flows of 40 c.f.h. argonthrough a /s inch orifice for a coating operation according to theinvention with the work-in-circuit, has been found to result insubstantially no alloying of the coating with the base metal, whereasthe employment of high currents, of the order of 140-160 amperes at thelower traverse speeds employed for the deoxidation and fusionapplications, results in gross melting of the base and in substantialdilution of the coating material and the base metal.

The following table sets forth various operating conditions obtained inproducing coatings of tungsten carbide-8 percent cobalt material (325 xD mesh) for a /z inch diameter round bar plain steel both rotated andtraversed under the various torches of the stated figures of thedrawing.

dioxide, nitrogen and the like may alternatively be employed. One itemwhich must be considered in the choice of a given torch gas is thatprecautions must be taken to prevent damage to the torch itself byreaction with the torch gas.

The following examples describe operation of the present inventionwherein gas mixtures are used as the torch gas.

In one example, a metallic tungsten coating was deposited employingapparatus similar to the type shown in FIG. 1. An are between the stickelectrode and the nozzle electrode was maintained at 190 amperes and 75volts. A mixture of 12 e.f.h. hydrogen and 138 c.f.h. argon passed downalong the tungsten cathode in the annular space between the shieldinggas sleeve and the cathode. A 150 c.f.h. argon stream carrying 50 grams/min. finely-divided tungsten powder (average particle size of 6.8microns) was introduced along the annular space surrounding theshielding gas sleeve. The total gas and powder flow passed concurrentlythrough the nozzle passage where they were heated by the collimated andwallstabilized arc and then passed through a nozzle extension tube. Thehot tungsten particles were then impinged on a /2 inch diameter brasstube workpiece positioned near the outlet of the nozzle extension tubeto form a dense adherent coating. The 0.050 inch thick as-depositedcoating had a modulus of rupture of 50,000 p.s.i. and a modulus ofelasticity of 22x10 p.s.i.

In another example using similar apparatus, an arc of 205 amperes and 65volts was maintained between the stick electrode and the nozzleelectrode. A mixture of 4 c.f.h. nitrogen and 150 c.f.h. argon passeddown along the tungsten stick cathode in the annular space between theshielding gas sleeve and the cathode. A 150 c.f.h. argon stream carrying28.3 grams/min. finely-divided tungsten powder was introduced along theannular space surrounding the shielding gas sleeve. The total gas andpowder flow passed concurrently through the nozzle passage where theywere heated by the collimated and wallstabilized arc and then passedthrough a nozzle extension tube. The hot tungsten particles were thenimpinged on a /2 inch diameter brass tube workpiece to form a denseadherent coating 0.050 inch thick.

It has been found that the speed of traverse of the workpiece requiredfor satisfactory coating operations is affected by the electric arccurrents and gas velocity employed in the process primarily when thework is in the Such coatings were all dense and well bonded to theworkpiece. They were also composed of irregular shaped microscopicleaves interlocked with each other.

The workpiece on which the arc-heated coating particles are deposited isheated by the hot gas efliuent from the torch.

When the workpiece is in the arc circuit, it is heated not only by thehigh thermal emuent but also by the are current. Such heating of theworkpiece can, to some extent, be overcome or compensated byinterrupting the application of coating from time to time and permittingthe workpiece to cool with or without directing a blast of cooling fluidsuch as air against it. A high traverse rate of the workpiece is alsodesirable. In transferred arc operation, the arc may desirably beinterrupted to cool the Workpiece if necessary. External cooling duringcoating application with a liquid spray or fog, such as liquid carbondioxide, can also be used provided the cooling fluid is not applieddirectly to the arc-efiiuent zone. Internal water cooling might also beused with hollow workpieces. According to the invention particles of amaterial such as tungsten carbide can be applied securely to a workpiecehaving a substantially diiferent coefficient of thermal expansion, suchas steel, by cooling the workpiece as described above.

Coatings applied according to the present invention may be built up toany desired thickness by continuing the depositing operation until thedesired thickness is obtained.

What is claimed is:

1. Method of welding particles of powder together to form a densecoherent mass composed of irregularly shaped microscopic leaves weldedinto interlocking relation with one. another, which comprisesconcurrently maintaining a high pressure electric are between anonconsumable stick electrode and a second electrode spaced therefrom,passing a stream of gas in contact with said stick electrode to containsaid are, passing said arc-containing gas stream through an orificewhich constricts the gas stream and wall-stabilizes a portion of saidare so as to collimate the energy of said are and gas stream and producea high pressure are and high thermal content effluent, passing powderedmaterial through and with said high thermal content erfiuent to producea high velocity stream of gas and heated particles, impinging said gasand heated particle stream against the surface of a suitable base,thereby depositing the so-heated particles on said 11 base as a denseadherent coherent mass wherein the soheated and deposited particles arewelded together.

2. Method of welding as defined by claim 1, wherein such powder is firstpassed through said are to take advantage of the extremely hightemperature afforded thereby.

Method of welding as defined by claim 1, wherein said base is inelectrical circuit with such high pressure are.

4. Method of welding as defined by claim 1, wherein said base iselectrically insulated from the arc circuit;

5. Method of welding as defined by claim 1, wherein said secondelectrode is provided with an orifice for constricting andwall-stabilizing such arcand gas stream.

6. Method of welding as defined by claim 5, wherein said base also is inelectrical circuit with such high pressure are.

7. Method of welding as defined by claim 1, wherein the value of thethermal energy supplied to the base is used to control the degree offusion between such welded particle mass and said base.

8. Method of depositing material on the surface of a workpiece,comprising concurrently maintaining a high pressure electric are betweena non-consumable stick electrode and a second electrode spacedtherefrom, passing a stream of gas in contact with said stick electrodeto contain said are, passing said arc-containing gas stream through anorifice which constricts the stream and wallstabilizes a portion of saidare so as to collimate the energy of said are and gas stream and producea high pressure are and high thermal content eifiuent, passing powderedmaterial through and with said high thermal content effiuent to producea high velocity stream of gas and heated particles, impinging said gasand heated particle stream against the surface of said workpiece, anddepositing the soheated particles on the surface of said workpiece.

9. Method as defined by claim 8, wherein such powdered material is firstpassed through such orifice with said are.

10. Method as defined by claim 9, wherein the current is conducted tosuch are through the workpiece.

11. Method as defined by claim 9, wherein said second electrode isprovided with an orifice for so-constricting and wall-stabilizing atleast a portion of such are and gas stream containing such powderedmaterial.

12. The method of depositing material on the surface of a workpiececomprising concurrently maintaining a high pressure electric arc of atleast amperes between a nonconsumable stick electrode and a secondnon-consumable electrode positioned therefrom and having an orifice atleast inch in diameter, passing a stream of gas hav ing a flow rate ofat least 20 c.f.h. around said stick electrode and through said orifice,whereby the arc-gas stream is collimated and a portion of the arc iswall-stabilized to form a high thermal content efiluent, positioning theoutlet of said orifice-containiug electrode about to 4 inches from suchsurface of said workpiece, passing coating material in the form ofpowder into said collimated high thermal content effluent, and directingthe resulting collimated hot particle stream against such surface ofsaid workpiece to deposit said powdered material thereon.

13. The method of depositing material on the surface of a workpiececomprising feeding a gas along a nonconsumable stick electrode, passingsaid gas through a secondary non-consumable electrode orifice and aprimary non-consumable electrode orifice, establishing a pilot arebetween said stick electrode and said secondary electrode, establishinga main are between said stick electrode and said primary electrode, aportion of said main are being Wall-stabilized, discharging a collimatedhigh thermal content efiluent from said primary electrode orifice,passing coating material in the form of powder into said high thermalcontent collimated effluent and directing the resulting collimatedstream containing hot coating material particles against such surface ofsaid workpieceto deposit said material thereon.

14-. Method of depositing material as defined by claim 8, wherein thegas is selected from the class consisting of argon, helium, nitrogen,hydrogen and mixtures thereof and the powdered material is selected fromthe class consisting of tungsten, molybdenum, tantalum, columbium andrhenium.

15. Method of depositing material as defined by claim 12 wherein the gasis selected from the class consisting of argon, helium, nitrogen,hydrogen and'mixtures thereof and the powdered ma erial is selected fromthe class consisting of tungsten, molybdenum, tantalum, columbium andrhenium.

References Cited in the file of this patent" UNITED STATES PATENTS1,243,795 Apple Oct. 23, 1917 1,395,269 Gebauer Nov. 1, 1921 2,149,656Armstrong et a1. Mar. 7, 1939 2,768,279 Rava Oct. 23, 1956

1. METHOD OF WELDING PARTICLES OF POWER TOGETHER TO FORM A DENSECOHERENT MASS COMPOSED OF IRREGULARLY SHAPED MICROSCOPIC LEAVES WELDEDINOT INTERLOCKING RELATION WITH ONE ANOTHER, WHICH COMPRISESCONCURRENTLY MAINTAINING A HIGH PRESSURE ELECTRIC ARC BETWEEN ANONCONSUMABLE STICK ELECTRODE AND A SECOND ELECTRODE SPACED THEREFROM,PASSING A STREAM OF GAS IN CONTACT WITH SAID STICK ELECTRODE TO CONTAINSAID ARC, PASSING SAID ARC-CONTAINING GAS STREAM THROUGH AN ORIFICEWHICH CONSTRICTS THE GAS STREAM AND WALL-STABILIZES A PORTION OF SAIDARC SO AS TO COLLIMATE THE ENERGY OF SAID ARC AND GAS STREAM AND PRODUCEA HIGH PRESSURE ARC AND HIGH THERMAL CONTENT EFFLUENT, PASSING POWDEREDMATERIAL THROUGH AND WITH SAID HIGH THERMAL CONTENT EFFLUENT TO PRODUCEA HIGH VELOCITY STREAM OF GAS AND HEATED PARTICLES, INPRINGING SAID GASAND HEATED PARTICLE STREAM AGAINST THE SURFACE OF A SUITABLE BASE,THEREBY DEPOSITING THE SO-HEATED PARTICLES ON SAID BASE AS A DENSEADHERENT COHERENT MASS WHEREIN THE SOHEATED AND DEPOSITED PARTICLES AREWELDED TOGETHER.